Essential Hypertension: This is the most common cause of high blood pressure in adults, and the patient's age and lack of other symptoms make it a likely diagnosis. The back pain could be unrelated or secondary to the hypertension.

Chronic Kidney Disease (CKD): CKD can cause hypertension, and back pain could be related to kidney issues such as polycystic kidney disease or kidney stones.
Sleep Apnea: Although the patient has a normal BMI, sleep apnea can still occur and is associated with hypertension. Back pain could be secondary to sleep disturbances.
Primary Aldosteronism: A condition where the adrenal glands produce too much aldosterone, leading to high blood pressure. Back pain is not a typical symptom but could be coincidental.

Pheochromocytoma: A rare tumor of the adrenal gland that can cause episodic or sustained hypertension. Back pain could be related to the tumor itself or to other symptoms like abdominal pain.
Aortic Dissection: A life-threatening condition where the aorta tears, which can cause severe back pain and hypertension. Although less likely, it is critical not to miss this diagnosis.
Renal Artery Stenosis: Narrowing of the arteries that supply blood to the kidneys, which can cause hypertension. Back pain could be related to the underlying cause of the stenosis.

Cushing's Syndrome: A condition caused by having too much cortisol in the body, which can lead to hypertension. Back pain could be related to osteoporosis, a complication of Cushing's syndrome.
Hyperthyroidism: Although more commonly associated with weight loss and other symptoms, hyperthyroidism can cause hypertension. Back pain could be secondary to muscle weakness or osteoporosis.
Coarctation of the Aorta: A congenital condition where the aorta is narrowed, which can cause hypertension. Back pain could be related to the condition itself or to associated anomalies.